Raked bar screen

ABSTRACT

A raked bar screen for a storm overflow comprises a grid of parallel spaced-apart bars (1) attached at their ends to a framework (2,3,14). A comb assembly (4,5,6) has teeth (5) which engage in the spaces between the grid bars (1). The comb assembly is longitudinally reciprocable, whereby the comb teeth can sweep longitudinally back and forth between the grid bars. The grid bars are shaped to provide a straight central portion (10) and longitudinal end portions (11, 12) which are angled in relation to the straight portion. In use, sweeping of the comb teeth between the grid bars at the angled end portions drives out entrapped solid matter.

This invention relates to a raked bar screen for a storm overflow.

Typical sewage systems are designed to accommodate not only normal flowsof waste water but also flows of storm water resulting from heavyrainfall, flooding and the like. In a typical system, a storm wateroverflow is positioned alongside a sewage pipe. In storm conditions, thelevel of water in the pipe rises and flows up through a horizontal barscreen and out over a weir for discharge into a receiving watercourse.In some installations, the bar screen is arranged vertically. Thepurpose of the bar screen is to filter out floating solid matter fromthe waste water and retain it in the sewage pipe, so that the receivingwatercourse does not become polluted. To prevent blockage of the barscreen, comb-like devices have been designed to continuously sweep thelongitudinal spaces between individual bars of the bar screen duringstorm conditions.

DE-A-4 215 002 discloses a raked bar screen for a storm overflow. Thescreen comprises a grid of parallel spaced-apart bars attached at theirends to a framework. A comb assembly is provided with teeth which engagein the spaces between the grid bars. The comb assembly is pivoted abouta central point above the grid and is caused to swing to and fro by adrive unit. The comb teeth accordingly sweep longitudinally back andforth between the grid bars. While this system satisfactorily keeps thecentral part of the grid free from solid blockage, it tends to sweepsolid matter to each longitudinal end of the grid. As this solid matterbuilds up at each end, the amplitude of swing of the comb assembly isreduced. Not only is the effectiveness of filtration of the screenreduced, but mechanical stresses build up in the linkage with the driveunit, and can lead to breakdown.

GB-A-2 310 382 discloses a sewage screen having at least one pocketextending downstream of at least some portions of the screen, inrelation to an intended direction of flow, to accommodate inorganicdebris. Portions of the screen may slant upwardly at the ends. However,the screen is arranged in an upwardly extending overflow shaft, so solidmaterial tends to accumulate in the pockets rather than being sweptalong in the flow of the sewage conduit.

The present invention provides a solution to the foregoing problems.

According to the invention, there is provided a raked bar screen for astorm overflow, comprising a grid of parallel spaced-apart bars attachedat their ends to a framework; a comb assembly having teeth adapted toengage in the spaces between the grid bars, the comb assembly beinglongitudinally reciprocable, whereby the comb teeth can sweeplongitudinally back and forth between the grid bars from end to end; thegrid bars being shaped to provide a straight central portion andlongitudinal end portions which are angled in relation to the straightportion, whereby in use sweeping of the comb teeth between the grid barsat the angled end portions will drive out entrapped solid matter; theframework including a solid cap portion joined to the ends of the barsand extending transversely thereto, and having a flat underside; and thecomb teeth each having leading and trailing edges, a flat top edge and abottom edge, the comb teeth being positioned such that, at theirextremity of travel, the leading edges protrude through the angled endportions of the bars, and the top edges contact and slide at leastpartly along the underside of the solid cap portion.

The grid is preferably arranged substantially horizontally. The endportions of the grid bars are preferably upturned at an acute angle awhich is less than the acute angle B formed between the leading andtrailing edges of the comb teeth with the central portion of the gridbars. At their extremity of travel, the leading edges of the comb teethprotrude through the angled end portions of the grid bars.

The solid cap portion which supports the ends of the grid barspreferably incorporates a knife edge arranged near each end of the gridbars so as to scrape adhered solid matter off the top edge of the combteeth at their extremity of travel.

The raked bar screen preferably comprises two or more banks of grid barsin tandem, each being separated from the next by a solid cap portion,and each being longitudinally swept from end to end by a respective combassembly.

The invention also provides a combined sewer overflow installation,comprising a sewage conduit, and a storm overflow conduit adjacent to atleast part of the sewage conduit, wherein part of the sewage conduit isin communication with part of the storm overflow conduit via an overflowweir, and wherein a raked bar screen as defined above is arrangedsubstantially horizontally above the part of the sewage conduit which isin communication with the storm overflow conduit, the grid bars being atsubstantially the same level as the top of the weir.

Reference is now made to the accompanying drawings, in which:

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of a raked bar screen according to a preferredembodiment of the invention;

FIG. 2 is a side view in the form of a section on the line 2--2 of FIG.1;

FIG. 3 is an end view in the form of a section on the line 3--3 of FIG.1;

FIG. 4 is a detailed side view of part of the apparatus at full travelof the comb assembly;

FIG. 5 is a plan view of a combined sewer overflow installation; and

FIG. 6 is a transverse section on the line 6--6 of FIG. 5.

The drawings show a double raked bar screen in which two sets of gridbars are arranged end-to-end. This forms a single module, which can bejoined with other modules if needed. The invention is, however, equallyapplicable to single grid bar systems and multiple grid bar systems. Asshown in the drawings, two grids are each formed by an assembly ofparallel spaced-apart grid bars I attached at their ends to a framework.The grid is arranged horizontally above a sewage conduit (as shown inFIGS. 5 and 6) so that, in storm conditions, as the water level risesthe overflow of water is filtered by the grid. The framework compriseslongitudinal lateral supports 2 and upstanding end supports 3.

A comb assembly comprises two transverse support bars 4, raised abovethe grid bars 1, and each supporting a line of underhanging comb teeth 5of plastics material. The teeth 5 are mounted on the transverse supportbar 4 in a spacing which corresponds to the spacing of the grid bars 1,and the teeth 5 engage respectively in the longitudinal spaces betweensuccessive grid bars 1. The transverse support bars 4 are linked by twolongitudinal support bars 6. The two transverse support bars 4 cooperaterespectively with the two sets of grid bars 1, so that the two sets ofcomb teeth 5 sweep the longitudinal spaces in the two arrays of gridbars 1, respectively. This is achieved by longitudinal reciprocation ofthe comb assembly. One of the transverse support bars 4 is joined by alinkage 7 to a hydraulic cylinder 8, mounted on a longitudinal supportbar 9, mounted between the respective end members 3 of the supportingframework. The hydraulic cylinder 8 is driven in reciprocating fashionon the support bar 9 by a supply of hydraulic fluid (not shown). Thecylinder 8 in turn drives the comb assembly. The system is switched onand off in response to water level sensors.

As shown in FIG. 2, each of the grid bars I comprises a straight centralportion 10 and an upturned portion 11,12 at each end. The end portions11, 12 are upturned at an acute angle α in relation to the straightcentral portion 10. The leading and trailing edges 13 of the comb teeth5 form an acute angle β with the central portion 10 of the grid bars.The angle β is greater than the angle α. At the extremity of travel ofthe comb teeth, shown in FIG. 4, the leading edge 13 of each comb toothpasses completely through the upturned end portion of the grid bars soas to protrude through to the other side. Together with the differencein the angles β and α, this ensures that solid matter entrapped betweenthe grid bars is completely swept out by the comb teeth.

The longitudinal end of each set of grid bars is joined to atransversely extending solid cap 14, which terminates in a knife edge14A adjacent the upper end of each upturned end portion of the gridbars, with the tip of the knife edge directed towards the centralportion. The knife edge 14A is arranged to overlap with part of an uppersurface 15 of the comb teeth 5. As a result, any solid matter whichaccumulates on the upper surface 15 is scraped off by the knife edge 14at full travel of the comb teeth. The upper surface 15 of the comb teethcontacts and slides at least partly along the underside of the solid cap14. As the grid bars are at the same level as the top of the weir (FIG.6), solid material swept from between the bars by the comb teeth isimmediately carried along by the flow of water in the sewage pipe.

FIGS. 5 and 6 show a sewage pipe 20 and storm overflow pipe 21 in aregion where they communicate with each other. A weir 22 is arrangedbetween them. A vertical baffle 23 above the sewer constrains overflowwater to pass through a horizontally arranged raked bar screen (asdescribed above). The raked bar screen is positioned between thevertical baffle 23 and the weir 22. The grid bars are at the same levelas the top of the weir 22.

What is claimed is:
 1. A raked bar screen for a storm overflow,comprising a grid of parallel spaced-apart bars attached at their endsto a framework; a comb assembly having teeth adapted to engage in thespaces between the grid bars, the comb assembly being longitudinallyreciprocable, whereby the comb teeth can sweep longitudinally back andforth between the grid bars from end to end; the grid bars being shapedto provide a straight central portion and longitudinal end portionswhich are upturned and angled in relation to the straight portion,whereby in use sweeping of the comb teeth between the grid bars at theangled end portions will drive out entrapped solid matter; the frameworkincluding a solid cap portion (14) joined to the ends of the bars (1)and extending transversely thereto, and having a flat underside; and thecomb teeth each having leading (13) and trailing edges, a flat top edge(15) and a bottom edge, the comb teeth being positioned Such that, attheir extremity of travel, the leading edges (13) protrude through theangled end portions of the bars, and the top edges contact and slide atleast partly along the underside of the solid cap portion.
 2. A rakedbar screen according to claim 1, in which the grid is arrangedsubstantially horizontally.
 3. A raked bar screen according to claim 2,in which the end portions of the grid bars are upturned at an acuteangle α which is less than the acute angle β formed between the leadingand trailing edges of the comb teeth with the central portion of thegrid bars.
 4. A raked bar screen according to claim 1, in which thesolid cap portion incorporates a knife edge 14A adjoining the ends ofthe grid bars so as to scrape adhered solid matter off the top edge ofthe comb teeth at their extremity of travel.
 5. A raked bar screenaccording to claim 1, comprising two or more banks of grid bars intandem, each being separated from the next by a solid cap portion, andeach being longitudinally swept from end to end by a respective combassembly.
 6. A combined sewer overflow installation, comprising a sewageconduit, and a storm overflow conduit adjacent to at least part of thesewage conduit, wherein part of the sewage conduit is in communicationwith part of the storm overflow conduit via an overflow weir, andwherein a raked bar screen according to claim 1 is arrangedsubstantially horizontally above the part of the sewage conduit which isin communication with the storm overflow conduit, the grid bars being atsubstantially the same level as the top of the weir.